General back light unit of liquid crystal display is constructed by a light introducing plate having dots for light diffusion which are printed on its reverse side, a light source (a cold-cathode tube or the like) arranged on one side or both sides of this light introducing plate, a light diffusing sheet superposed on this light introducing plate, a lens film (prism sheet) superposed on this light diffusing sheet or on above and below thereof, and the like.
As such light diffusing sheets, (1) a sheet in which a light diffusing layer containing polymer beads or inorganic particulates as a light diffusing agent is arranged on at least one side of a transparent base material (Japanese Patent No. 2,665,301), (2) a sheet in which an unevenness is formed on one side or both sides of a transparent plastic film by applying embossing thereto and a light diffusing layer containing particulates is also arranged on one side or both sides thereof (JP-A-11-337711), (3) a sheet in which a random irregularity is formed on the surface without containing a light diffusing agent (Japanese Patent No. 2,562,265) and the like are known.
However, the light diffusing sheet of (1) had problems such as insufficient visibility, reduction of displaying quality, high production cost due to poor yield and the like, because the polymer beads or inorganic particulates protruding from the surface of light diffusing layer impair the lens film superposed thereon and are apt to fallout from the light diffusing layer by an impact and the like.
Also, though dispersing ability of the light diffusing sheet of (2) is improved by the irregularity formed on one side or both sides thereof, it had the same problems as those of the light diffusing sheet of (1) because a light diffusing layer containing particulates is arranged on its surface and, in addition, it had another problem of further increasing its production cost because it requires two processes of a irregularity forming process by embossing and a light diffusing layer forming process.
In addition, the light diffusing sheet of (3) had a problem in that when the irregular shape formed on the surface of one side or both sides is not appropriate, the light scattering becomes insufficient or the light scattering becomes heterogeneous to cause partial variation of the brightness or becomes visualization of dots on the light introducing plate face. There was also a problem in that wrinkles are generated on the light diffusing sheet when it is heated by light and heat from the light source, thus resulting in uneven light scattering and varied brightness.
The invention has been made by taking the aforementioned problems into consideration, and an object thereof is to provide an easily producible light diffusing sheet which is rich in the amount of entering light from one side and can emit uniform diffused light with no variation of brightness from the opposite side by controlling optical loss at a small degree. Also, its another object is to provide a light diffusing sheet which does not generate wrinkles when heated by light and heat from the light source and can perform stable and uniform light scattering with less optical loss.